Make sure your Android device is plugged in via USB, and USB Debugging is enabled.

To enable USB Debugging:
Navigate to Settings > About Phone > Build number
Click 'Build number' 7 times
Now navigate back to Settings > Developer Options
Enable 'USB Debugging' and follow the prompts
You may also be prompted to allow access to your PC once your device is plugged in.

You can check if the device is connected by running:

third_party/android_tools/sdk/platform-tools/adb devices

which prints a list of connected devices. If not connected, try unplugging and reattaching your device.


src: https://www.chromium.org/developers/how-tos/android-build-instructions

Also see, with screenshots these slides explaining kinda the same thing:
https://docs.google.com/presentation/d/1r3VlhZIZVSufZlAeICJet6QBtyAF7z06_ysl1kUKME4/edit?pref=2&pli=1

